frontend: document runtime host bundle

frontend/runtime-host/README.md

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+# sgClaw Runtime Host Bundle
+
+`frontendBundleDir` lets SuperRPA prefer an external `sgclaw-chat` bundle at
+runtime while keeping the browser host, security boundary, and `chrome.send`
+bridge inside Chromium.
+
+## Contract
+
+- SuperRPA remains the host and security boundary.
+- sgClaw remains the runtime and planner/execution owner.
+- The frontend bundle only renders state that comes from the existing host
+  bridge.
+- Missing external files fall back to Chromium-bundled resources.
+
+## Directory Layout
+
+The external bundle root must mirror the Chromium resource paths under
+`functions/sgclaw-chat/`.
+
+Example:
+
+```text
+frontend-bundle/
+  sgclaw-chat/
+    sgclaw-chat.js
+    sgclaw-chat.css.js
+    sgclaw-chat.html.js
+    components/
+      sgclaw-chat-shell.js
+      sgclaw-message-list.js
+```
+
+When `frontendBundleDir` points at `frontend-bundle/`, the browser will try to
+serve `sgclaw-chat/*` from that directory first. Any file that is absent will
+continue loading from the built-in Chromium resource pack.
+
+## Packaging Notes
+
+- Keep relative import paths inside the bundle identical to the bundled
+  `sgclaw-chat` tree.
+- Do not move planner or execution logic into the bundle.
+- Treat [manifest.example.json](/home/zyl/projects/sgClaw/claw/frontend/runtime-host/manifest.example.json)
+  as a packaging reference for humans and tooling.